1 2 3 4 5 6 7 8 9 The Coast: es and Shoreline Processes Trujillo & Thurman, Chapter 10 Oceanography 101 Chapter Objectives Recognize the various landforms characteristic of beaches and coastal regions. Identify seasonal changes that beaches experience. Discuss how longshore currents are created and what longshore drift is. Note the origin of sediment for beaches and how the coastline responds to variations in supply. Describe how coastal features are formed by wave erosion and deposition. Chapter Objectives (continued) Understand local changes that occur in coastline elevation and explain observed trends in the relative position of sea level. Explain how climate change can affect the nature of the coastline. Recognize barrier island features and describe how barrier islands are formed and evolve. Identify the types of hard stablization and discuss the effects they have on shorelines. Overview Coastal region constantly changes Primarily due to waves Erosion Deposition Many people live in coastal regions 80% of people in U.S. live within easy access of coast Coastal regions Coast and coastline Shore, foreshore, backshore Nearshore, offshore profile Wave-worked sediments Wave-cut bench Recreational beach Berm face Longshore bars Longshore trough Composition of beaches Locally available material May be coarse or fine Boulders from local cliffs Sand from rivers Mud from rivers Significant biologic material at tropical beaches Example, Coral reef material Sand movement along beach Perpendicular to shoreline (toward and away) 1
10 11 12 13 14 15 16 17 18 19 Parallel to shoreline (up-coast or down-coast) Swash After wave breaks, uprush of water (swash) on beach Sediment moved toward land Backwash Water returns to ocean Sediment moved away from shore Light wave activity Swash dominates Sediment moved toward shore Wider beach Fair weather Summertime beach Heavy wave activity Backwash dominates Sediment moved away from shore Narrower beach Sand forms offshore sand bars Stormy weather Wintertime beach Wave refraction causes water and sand to move parallel to shore Zigzag motion in surf zone Longshore transport Longshore transport Millions of tons of sediment moved yearly Direction of transport changes due to wave approach In general, sediment transported southward along Atlantic and Pacific coasts of U.S. Erosional shorelines Well-developed cliffs Recent tectonic activity Headlands Wave-cut cliff with sea cave Sea arches Sea stacks Marine terrace Wave erosion increases with More shore exposed to open ocean Smaller tidal range Weaker bedrock Depositional shorelines Primarily deposited by longshore drift Spit 2
20 21 22 23 24 25 26 27 28 29 30 Bay barrier Tombolo Delta compartment Depositional shorelines s Long, narrow offshore deposits parallel to shore Most developed due to rise of sea level about 18,000 years ago Common East and Gulf coasts of U.S. Protect mainland from high wave activity Ocean beach Dunes Barrier flat High salt marsh Low salt marsh Lagoon Movement landward over time Associated with rising sea levels Older peat deposits found on ocean beach Deltas River sediments reworked by ocean processes: waves, tides Distributaries carry sediment to ocean compartments Rivers supply sediment Offshore submarine canyons drain sediments from beach starvation Emerging shorelines Shorelines above current sea level Marine terraces Submerging shorelines Shoreline below current sea level Drowned beaches Submerged dune topography Drowned river valleys (estuaries) Changing sea level Local tectonic processes Example, Pacific Coast of U.S. and active plate margin Isostatic adjustments Ice-loading Global (eustatic) changes in sea level Changes in seafloor spreading rates Lake buildup or destruction Ice volume changes Eustatic changes in sea level Ice build up (glaciation) Ice melting (deglaciation) 3
31 32 33 34 35 36 37 38 39 40 41 42 43 44 Thermal contraction and expansion of seawater About 120 m (400 ft) change in sea level Global warming and changing sea level About 0.6 o C (1.1 o F) warmer over last 130 years Sea level rose 10-15 cm (4-10 in) over past 100 years If global warming continues, higher sea level U.S. coasts Erosion or deposition dominates Type of bedrock Tidal range and wave exposure Active tectonics Eustatic changes in sea level Atlantic coast Most coasts open to wave attack s common Varied bedrock from resistant rocks to non-resistant sedimentary rocks Sea level rising about 0.3 m (1 ft) per century Drowned river valleys common Average erosion 0.8 m (2.6 ft) per year Atlantic coast s Drowned river valleys Gulf coast Low tidal range Generally low wave energy Tectonic subsidence Mississippi delta dominates Locally sea level rises due to compaction of delta sediments Average rate of erosion 1.8 m (6 ft) per year Pacific coast Tectonically rising Bedrock typically non-resistant sedimentary rocks Open exposure to high energy waves Average rate of erosion 0.005 m (0.016 ft) per year Hard stabilization Structures built to decrease coastal erosion and Interfere with sand movement Often results in unwanted outcomes Some structures may increase wave erosion Groins and groin fields Jetties Breakwaters Seawalls Groins and groin fields Jetties Breakwaters Seawalls Alternatives to hard stabilization Construction restrictions 4
45 46 Limit building near shorelines National Flood Insurance Program encouraged construction replenishment Sand added to beach/longshore current Alternatives to hard stabilization Relocation Move structures rather than protect them in areas of erosion End of CHAPTER 10 es and Shoreline Processes 5